Bug 1743020 - Part 3: Export mozilla::MoveOnlyFunction based on function2, r=glandium

A custom defintion wrapping fu2::function_base is used to customize the
inline buffer's size and alignment to make it compatible with nsTArray.
Without the custom wrapper, `alignof(max_align_t)` is used, which is
larger than nsTArray's max alignment on some platforms.

Differential Revision: https://phabricator.services.mozilla.com/D145691

js/src/make-source-package.py

@@ -176,6 +176,7 @@ rsync_filter_list = """
 
 + /.cargo/config.in
 
++ /third_party/function2/**
 - /third_party/python/gyp
 + /third_party/python/**
 + /third_party/rust/**

mfbt/MoveOnlyFunction.h

@@ -0,0 +1,47 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_MoveOnlyFunction_h
+#define mozilla_MoveOnlyFunction_h
+
+// Use stl-like empty propagation to avoid issues with wrapping closures which
+// implicitly coerce to bool.
+#define FU2_WITH_LIMITED_EMPTY_PROPAGATION
+
+#include "function2/function2.hpp"
+
+namespace mozilla {
+
+/// A type like `std::function`, but with support for move-only callable
+/// objects.
+///
+/// A similar type is proposed to be added to the standard library as
+/// `std::move_only_function` in C++23.
+///
+/// Unlike `std::function`, the function signature may be given const or
+/// reference qualifiers which will be applied to `operator()`. This can be used
+/// to declare const qualified or move-only functions.
+///
+/// The implementation this definition depends on (function2) also has support
+/// for callables with overload sets, however support for this was not exposed
+/// to align better with the proposed `std::move_only_function`, which does not
+/// support overload sets.
+///
+/// A custom typedef over `fu2::function_base` is used to control the size and
+/// alignment of the inline storage to store 2 aligned pointers, and ensure the
+/// type is compatible with `nsTArray`.
+template <typename Signature>
+using MoveOnlyFunction = fu2::function_base<
+    /* IsOwning */ true,
+    /* IsCopyable */ false,
+    /* Capacity */ fu2::capacity_fixed<2 * sizeof(void*), alignof(void*)>,
+    /* IsThrowing */ false,
+    /* HasStrongExceptionGuarantee */ false,
+    /* Signature */ Signature>;
+
+}  // namespace mozilla
+
+#endif  // mozilla_MoveOnlyFunction_h

mfbt/moz.build

@@ -71,6 +71,7 @@ EXPORTS.mozilla = [
     "MaybeStorageBase.h",
     "MemoryChecking.h",
     "MemoryReporting.h",
+    "MoveOnlyFunction.h",
     "NonDereferenceable.h",
     "NotNull.h",
     "Opaque.h",
@@ -133,6 +134,10 @@ EXPORTS["double-conversion"] = [
     "double-conversion/double-conversion/utils.h",
 ]
 
+EXPORTS.function2 += [
+    "/third_party/function2/include/function2/function2.hpp",
+]
+
 LOCAL_INCLUDES += [
     "/mfbt/double-conversion",
 ]